Cellulose fiber and derivatives are widely used in paper, absorbent products, food or food-related applications, pharmaceuticals, and in industrial applications. The main sources of cellulose fiber are wood pulp and cotton. The cellulose source and the cellulose processing conditions generally dictate the cellulose fiber characteristics, and therefore, the fiber's applicability for certain end uses. A need exists for cellulose fiber that is relatively inexpensive to process, yet is highly versatile, enabling its use in a variety of applications.
Cellulose exists generally as a polymer chain comprising hundreds to tens of thousands of glucose units. Various methods of oxidizing cellulose are known. In cellulose oxidation, hydroxyl groups of the glycosides of the cellulose chains can be converted, for example, to carbonyl groups such as aldehyde groups or carboxylic acid groups. Depending on the oxidation method and conditions used, the type, degree, and location of the carbonyl modifications may vary. It is known that, certain oxidation conditions may degrade the cellulose chains themselves, for example by cleaving the glycosidic rings in the cellulose chain, resulting in depolymerization. In most instances, depolymerized cellulose not only has a reduced viscosity, but also has a shorter fiber length than the starting cellulosic material. When cellulose is degraded, such as by depolymerizing and/or significantly reducing the fiber length and/or the fiber strength, it may be difficult to process and/or may be unsuitable for many downstream applications. A need remains for methods of modifying cellulose fiber that may improve both carboxylic acid and aldehyde functionalities, which methods do not extensively degrade the cellulose fiber. This disclosure provides unique methods that resolve one or more of these deficiencies.
Various attempts have been made to oxidize cellulose to provide both carboxylic and aldehydic functionality to the cellulose chain without degrading the cellulose fiber. In traditional cellulose oxidation methods, it may be difficult to control or limit the degradation of the cellulose when aldehyde groups are present on the cellulose. Previous attempts at resolving these issues have included the use of multi-step oxidation processes, for instance site-specifically modifying certain carbonyl groups in one step and oxidizing other hydroxyl groups in another step, and/or providing mediating agents and/or protecting agents, all of which may impart extra cost and by-products to a cellulose oxidation process. Thus, there exists a need for methods of modifying cellulose that are cost effective and/or can be performed in a single step of a process, such as a kraft process.
This disclosure provides novel methods that offer vast improvements over methods attempted in the prior art. Generally, oxidization of cellulose kraft fibers, in the prior art, is conducted after the bleaching process. Surprisingly, the inventors have discovered that it is possible to use the existing stages of a bleaching sequence, particularly the fourth stage of a five stage bleaching sequence, for oxidation of cellulose fibers. Furthermore, surprisingly, the inventors have discovered that a metal catalyst, particularly an iron catalyst, could be used in the bleaching sequence to accomplish this oxidation without interfering with the final product, for example, because the catalyst did not remain bound in the cellulose resulting in easier removal of at least some of the residual iron prior to the end of the bleaching sequence than would have been expected based upon the knowledge in the art. Moreover, unexpectedly, the inventors have discovered that such methods could be conducted without substantially degrading the fibers.
It is known in the art that cellulose fiber, including kraft pulp, may be oxidized with metals and peroxides and/or peracids. For instance, cellulose may be oxidized with iron and peroxide (“Fenton's reagent”). See Kishimoto et al., Holzforschung, vol. 52, no. 2 (1998), pp. 180-184. Metals and peroxides, such as Fenton's reagent, are relatively inexpensive oxidizing agents, making them somewhat desirable for large scale applications, such as kraft processes. In the case of Fenton's reagent, it is known that this oxidation method can degrade cellulose under acidic conditions. Thus, it would not have been expected that Fenton's reagent could be used in a kraft process without extensive degradation of the fibers, for example with an accompanying loss in fiber length, at acidic conditions. To prevent degradation of cellulose, Fenton's reagent is often used under alkaline conditions, where the Fenton reaction is drastically inhibited. However, additional drawbacks may exist to using Fenton's reagent under alkaline conditions. For example, the cellulose may nonetheless be degraded or discolored. In kraft pulp processing, the cellulose fiber is often bleached in multi-stage sequences, which traditionally comprise strongly acidic and strongly alkaline bleaching steps, including at least one alkaline step at or near the end of the bleaching sequence. Therefore, contrary to what was known in the art, it was quite surprising that fiber oxidized with iron in an acidic stage of a kraft bleaching process could result in fiber with enhanced chemical properties, but without physical degradation or discoloration.
Thus there is a need for a low cost and/or single step oxidation that could impart both aldehyde and carboxylic functionalities to a cellulose fiber, such as a fiber derived from kraft pulp, without extensively degrading the cellulose and/or rendering the cellulose unsuitable for many downstream applications. Moreover, there remains a need for imparting high levels of carbonyl groups, such as carboxylic acid, ketone, and aldehyde groups, to cellulose fiber. For example, it would be desirable to use an oxidant under conditions that do not inhibit the oxidation reaction, unlike the use of Fenton's reagent at alkaline pH for instance, to impart high levels of carbonyl groups. The present inventors have overcome many difficulties of the prior art, providing methods that meet these needs.
In addition to the difficulties in controlling the chemical structure of cellulose oxidation products, and the degradation of those products, it is known that the method of oxidation may affect other properties, including chemical and physical properties and/or impurities in the final products. For instance, the method of oxidation may affect the degree of crystallinity, the hemi-cellulose content, the color, and/or the levels of impurities in the final product. Ultimately, the method of oxidation may impact the ability to process the cellulose product for industrial or other applications.
Bleaching of wood pulp is generally conducted with the aim of selectively increasing the whiteness or brightness of the pulp, typically by removing lignin and other impurities, without negatively affecting physical properties. Bleaching of chemical pulps, such as kraft pulps, generally requires several different bleaching stages to achieve a desired brightness with good selectivity. Typically, a bleaching sequence employs stages conducted at alternating pH ranges. This alternation aids in the removal of impurities generated in the bleaching sequence, for example, by solubilizing the products of lignin breakdown. Thus, in general, it is expected that using a series of acidic stages in a bleaching sequence, such as three acidic stages in sequence, would not provide the same brightness as alternating acidic/alkaline stages, such as acidic-alkaline-acidic. For instance, a typical DEDED sequence produces a brighter product than a DEDAD sequence (where A refers to an acid treatment). Accordingly, a sequence that does not have an intervening alkaline stage, yet produces a product with comparable brightness, would not be expected by a person of skill in the art.
Generally, while it is known that certain bleaching sequences may have advantages over others in a kraft process, the reasons behind any advantages are less well understood. With respect to oxidation, no studies have shown any preference for oxidation in a particular stage of a multi-stage sequence or any recognition that fiber properties can be affected by post oxidation stages/treatments. For instance, the prior art does not disclose any preference for a later stage oxidation over an earlier stage oxidation. In some embodiments, the disclosure provides methods uniquely performed in particular stages (e.g., later stages of a bleaching process) that have benefits in the kraft process and that result in fibers having a unique set of physical and chemical characteristics.
In addition, with respect to brightness in a kraft bleaching process, it is known that metals, in particular transition metals naturally present in the pulp starting material, are detrimental to the brightness of the product. Thus, bleaching sequences frequently aim to remove certain transition metals from a final product to achieve a target brightness. For example, chelants may be employed to remove naturally occurring metal from a pulp. Thus, because there is emphasis on removing the metals naturally present in the pulp, a person of skill in the art would generally not add any metals to a bleaching sequence as that would compound the difficulties in achieving a brighter product.
With respect to iron, moreover, addition of this material to a pulp leads to significant discoloration, akin to the discoloration present when, for example, burning paper. This discoloration, like the discoloration of burnt paper, has heretofore been believed to be non-reversible. Thus, it has been expected that upon discoloring a wood pulp with added iron, the pulp would suffer a permanent loss in brightness that could not be recovered with additional bleaching.
Thus, while is known that iron or copper and peroxide can inexpensively oxidize cellulose, heretofore they have not been employed in pulp bleaching processes in a manner that achieves a comparable brightness to a standard sequence not employing an iron or copper oxidation step. Generally, their use in pulp bleaching processes has been avoided. Surprisingly, the inventors have overcome these difficulties, and in some embodiments, provide a novel method of inexpensively oxidizing cellulose with iron or copper in a pulp bleaching processes. In some embodiments, the methods disclosed herein result in products that have characteristics that are very surprising and contrary to those predicted based on the teachings of the prior art. Thus, the methods of the disclosure may provide products that are superior to the products of the prior art and can be more cost-effectively produced.
For instance, it is generally understood in the art that metals, such as iron, bind well to cellulose and cannot be removed by normal washing. Typically, removing iron from cellulose is difficult and costly, and requires additional processing steps. The presence of high levels of residual iron in a cellulose product is known to have several drawbacks, particularly in pulp and papermaking applications. For instance, iron may lead to discoloration of the final product and/or may be unsuitable for applications in which the final product is in contact with the skin, such as in diapers and wound dressings. Thus, the use of iron in a kraft bleaching process would be expected to suffer from a number of drawbacks.
Heretofore, oxidation treatment of kraft fiber to improve functionality has often been limited to oxidation treatment after the fiber was bleached. Moreover, known processes for rendering a fiber more aldehydic also cause a concomitant loss in fiber brightness or quality. Furthermore, known processes that result in enhanced aldehydic functionality of the fiber also result in a loss of carboxylic functionality. The methods of this disclosure do not suffer from one or more of those drawbacks.
Kraft fiber, produced by a chemical kraft pulping method, provides an inexpensive source of cellulose fiber that generally maintains its fiber length through pulping, and generally provides final products with good brightness and strength characteristics. As such, it is widely used in paper applications. However, standard kraft fiber has limited applicability in downstream applications, such as cellulose derivative production, due to the chemical structure of the cellulose resulting from standard kraft pulping and bleaching. In general, standard kraft fiber contains too much residual hemi-cellulose and other naturally occurring materials that may interfere with the subsequent physical and/or chemical modification of the fiber. Moreover, standard kraft fiber has limited chemical functionality, and is generally rigid and not highly compressible.
The rigid and coarse nature of kraft fiber can require the layering or addition of different types of materials, such as cotton, in applications that require contact with human skin, for example, diapers, hygiene products, and tissue products. Accordingly, it may be desirable to provide a cellulose fiber with better flexibility and/or softness to reduce the requirement of using other materials, for example, in a multi-layered product.
Cellulose fiber in applications that involve absorption of bodily waste and/or fluids, for example, diapers, adult incontinence products, wound dressings, sanitary napkins, and/or tampons, is often exposed to ammonia present in bodily waste and/or ammonia generated by bacteria associated with bodily waste and/or fluids. It may be desirable in such applications to use a cellulose fiber which not only provides bulk and absorbency, but which also has odor reducing and/or antibacterial properties, e.g., can reduce odor from nitrogenous compounds, such as ammonia (NH3). Heretofore, modification of kraft fiber by oxidation to improve its odor control capability invariably came with an undesirable decrease in brightness. A need exists for an inexpensive modified kraft fiber that exhibits good absorbency characteristics and/or odor control capabilities while maintaining good brightness characteristics.
In today's market, consumers desire absorbent products, for example, diapers, adult incontinence products, and sanitary napkins, that are thinner. Ultra-thin product designs require lower fiber weight and can suffer from a loss of product integrity if the fiber used is too short. Chemical modification of kraft fiber can result in loss of fiber length making it unacceptable for use in certain types of products, e.g., ultra-thin products. More specifically, kraft fiber treated to improve aldehyde functionality, which is associated with improved odor control, may suffer from a loss of fiber length during chemical modification making it unsuitable for use in ultra-thin product designs. A need exists for an inexpensive fiber that exhibits compressibility without a loss in fiber length which makes it uniquely suited to ultra-thin designs (i.e., the product maintains good absorbency based upon the amount of fiber that can be compressed into a smaller space while maintaining product integrity at lower fiber weights).
Traditionally, cellulose sources that were useful in the production of absorbent products or tissue were not also useful in the production of downstream cellulose derivatives, such as cellulose ethers and cellulose esters. The production of low viscosity cellulose derivatives from high viscosity cellulose raw materials, such as standard kraft fiber, required additional manufacturing steps that would add significant cost while imparting unwanted by-products and reducing the overall quality of the cellulose derivative. Cotton linter and high alpha cellulose content sulfite pulps, which generally have a high degree of polymerization, are generally used in the manufacture of cellulose derivatives such as cellulose ethers and esters. However, production of cotton linters and sulfite fiber with a high degree of polymerization and/or viscosity is expensive due to the cost of the starting material, in the case of cotton; the high energy, chemical, and environmental costs of pulping and bleaching, in the case of sulfite pulps; and the extensive purifying processes required, which applies in both cases. In addition to the high cost, there is a dwindling supply of sulfite pulps available to the market. Therefore, these fibers are very expensive, and have limited applicability in pulp and paper applications, for example, where higher DP or higher viscosity pulps may be required. For cellulose derivative manufacturers these pulps constitute a significant portion of their overall manufacturing cost. Thus, there exists a need for low cost fibers, such as a modified kraft fiber, that may be used in the production of cellulose derivatives.
There is also a need for inexpensive cellulose materials that can be used in the manufacture of microcrystalline cellulose. Microcrystalline cellulose is widely used in food, pharmaceutical, cosmetic, and industrial applications, and is a purified crystalline form of partially depolymerized cellulose. The use of kraft fiber in microcrystalline cellulose production, without the addition of extensive post-bleaching processing steps, has heretofore been limited. Microcrystalline cellulose production generally requires a highly purified cellulosic starting material, which is acid hydrolyzed to remove amorphous segments of the cellulose chain. See U.S. Pat. No. 2,978,446 to Battista et al. and U.S. Pat. No. 5,346,589 to Braunstein et al. A low degree of polymerization of the chains upon removal of the amorphous segments of cellulose, termed the “level-off DP,” is frequently a starting point for microcrystalline cellulose production and its numerical value depends primarily on the source and the processing of the cellulose fibers. The dissolution of the non-crystalline segments from standard kraft fiber generally degrades the fiber to an extent that renders it unsuitable for most applications because of at least one of 1) remaining impurities; 2) a lack of sufficiently long crystalline segments; or 3) it results in a cellulose fiber having too high a degree of polymerization, typically in the range of 200 to 400, to make it useful in the production of microcrystalline cellulose. Kraft fiber having good purity and/or a lower level-off DP value, for example, would be desirable, as the draft fiber may provide greater versatility in microcrystalline cellulose production and applications.
In the present disclosure, fiber having one or more of the described properties can be produced simply through modification of a typical kraft pulping plus bleaching process. Fiber of the present disclosure overcomes many of the limitations associated with known modified kraft fiber discussed above.